I had the same noise on my GTS. It only seems to happen when the car slows to a halt in a queue. I initially thought i might be brakes or air suspension as it occurs when the heating and air con are off.
I had it investigated at Guildford in March, who did not appear to have heard this noise before. I was told that it might be the air conditioning as Porsche had recently changed the unit on the Macan and they would contact the factory.
The next day they called back and confirmed this noise is from the air-conditioning and that this is normal.
I guess Porsche think that if we buy a car with a button on the centre console which makes the exhaust louder, a little warbling won't bother us!
Technical Forum for the Porsche Macan